Estimation of unitary quantum operations
Abstract
The problem of optimally estimating an unknown unitary quantum operation with the aid of entanglement is addressed. The idea is to prepare an entangled pair, apply the unknown unitary to one of the two parts, and then measure the joint output state. This measurement could be an entangled one or it could be separable (e.g., measurements which can be implemented with local operations and classical communication or LOCC). A comparison is made between these possibilities and it is shown that by using nonseparable measurements one can improve the accuracy of the estimation by a factor of 2(d+1)/d where d is the dimension of the Hilbert space on which U acts.
 Publication:

Physical Review A
 Pub Date:
 February 2004
 DOI:
 10.1103/PhysRevA.69.022303
 arXiv:
 arXiv:quantph/0305104
 Bibcode:
 2004PhRvA..69b2303B
 Keywords:

 03.67.a;
 Quantum information;
 Quantum Physics
 EPrint:
 6 pages. Revised version. Typos corrected. Some discussion added. Reference fixed